1. Field of the Invention
The present invention relates to a straight-through-repeater, and more particularly relates to a straight-through-repeater which relays frequency-modulated microwave signal.
2. Description of the Prior Art
With the progress in microwave semiconductor technology, a microwave straight-through-repeater has come into practical use. Such microwave semiconductor has been reported in, for example 7th EMC (Europe Microwave Conference) CS2, pages 382 through 386, entitled "A 7 GHz Band GaAs FET Amplifier For the Straight-Through-Repeater". This repeater relays a received microwave signal after direct amplification thereof without converting it into a conventional intermediate frequency signal. Therefore, it is not necessary for the repeater to possess a conventional local oscillator therein which converts the received microwave signal into the intermediate frequency signal. Thus, the repeater may be very simple in construction.
However, although the straight-through-repeater has the advantage that it is simple in construction, the repeater simultaneously has the disadvantages recited in following items (a) through (d).
(a) It is impossible to pick out a baseband signal from the received microwave signal. In a typical repeater, that is a non-straight-through-repeater, it is very easy to obtain the baseband signal from the received microwave signal, due to the presence of the intermediate frequency signal therein, by means of a simple intermediate frequency signal demodulator. Accordingly, in the straight-through-repeater, it is impossible to reproduce the so-called order wire signal therefrom, which order wire signal has to be provided in the repeater for use in emergencies, such as trouble with or an accident at a repeater station. Further, since the baseband signal can not be reproduced in the repeater, it is impossible to create another microwave communication route branched from the repeater.
(b) The straight-through-repeater is relatively high in cost. This is due to the fact that the aforesaid microwave semiconductor having a relatively high power amplifying capability, is presently still high in cost.
(c) The straight-through-repeater comprising the microwave semiconductor mentioned in the item (b), above, should also be provided with a very narrow microwave band filtering capability. However, it is difficult for this repeater to realize such filtering capability in a cheap and simple manner.
(d) The straight-through-repeater comprising the microwave semiconductor mentioned in the item (b), above, should also be provided with a so-called automatic gain controlling capability and/or amplitude limiting capability. However, it is difficult for this repeater to realize such automatic gain controlling capability and/or amplitude limiting capability in a cheap and simple manner.